A. Atherosclerosis and Lipoproteins
Atherosclerosis is the disease in which cholesterol and other lipids, accumulating on the walls of arteries, form bulky plaques that inhibit the flow of blood and may lead to the formation of a clot, obstructing an artery and causing occlusive thrombotic or embolic disease such as a heart attack or stroke. Up to 50 percent of all deaths in the United States are caused by atherosclerosis and its secondary complications.
Human atherosclerosis is defined as the accumulation of selected lipids, including cholesterol, and cells in the walls of arteries and with time produces occlusive lesions. Although the etiology of atherosclerosis is multi-factorial, a large body of clinical, pathologic, genetic and experimental evidence suggests that abnormalities of lipoprotein metabolism can contribute to the development of atherosclerosis. These lipids are carried in the blood stream as lipid-protein complexes called lipoproteins.
Atherosclerosis, and particularly that form known as coronary artery disease (CAD), is a major health problem. Atherosclerosis and its related vascular diseases accounted for 983,000 deaths in 1983; and CAD alone accounts for more deaths annually than all forms of cancer combined. In the United States, more than 1 million heart attacks occur each year and more than five hundred thousand people die as a result of this disease. In direct health care costs, CAD costs the United States more than $60 billion a year. This enormous toll has focused attention on ways to identify particular populations at risk for CAD so that the disease can be controlled with diet, behavioral modification (exercise), and specific therapeutic agents.
Four major classes of cholesterol-associated plasma lipoprotein particles have been defined, and have their origin in the intestine or liver. These particles are involved in the transport of the neutral lipids including cholesterol and triglycerides. All classes of plasma lipoproteins have apolipoproteins associated with the lipid-protein complex; and the apolipoproteins play requisite roles in the function of these lipoproteins.
The first class is the chylomicrons. They are the largest of the lipoproteins and are rich in triglycerides. The site of origin of the chylomicrons is the intestine.
Whereas apolipoproteins are a quantitatively minor proportion of the mass of chylomicrons, apolipoproteins A-I, A-II and A-IV are reportedly significantly associated with chylomicrons, and intestinal synthesis of these A apolipoproteins has been found. Chylomicrons also contain apolipoprotein B-48. Much of the chylomicron complement of A apolipoproteins is lost, and C and E apolipoproteins are acquired when chylomicrons are exposed to plasma or high density lipoprotein (HDL) in vitro. Intestinal production of the A apolipoproteins (apo A) may be regulated by factors other than fat absorption and chylomicron formation.
The next class of lipoproteins is the very low density lipoproteins, VLDL. The VLDL particle is made in the liver and is involved in triglyceride metabolism and transport of these lipids from the liver. The apolipoproteins apo B-100 and apo E are the major constituents of the VLDL particle.
The third lipoprotein is called low density lipoprotein (LDL), and is a specific product of the catabolism of VLDL. The predominant apolipoprotein in the LDL particle is apolipoprotein B-100, or apo B-100.
The results of the now classic Framingham study (1971) showed a clear correlation between risk for CAD and serum cholesterol levels. This study also demonstrated that elevated levels of low density lipoprotein (LDL) cholesterol are associated with increased risk of CAD. Recently, a study conducted by the Lipid Research Clinics Coronary Primary Prevention Trial (1984) has demonstrated that plasma levels of cholesterol and LDL cholesterol can be reduced by a combined regime of diet and drugs, and that this reduction of plasma cholesterol results in reduction of the incidence of CAD mortality.
LDL is the major cholesterol-carrying lipoprotein in plasma. LDL is a large spherical particle whose lipid core is composed of about 1500 molecules of cholesterol, each attached by an ester linkage to a long chain fatty acid. This core of cholesteryl esters is enclosed by a layer of phospholipid, unesterified cholesterol molecules, and a single molecule of apolipoprotein B-100. The phospholipids are arrayed so that the hydrophilic heads are on the outside, allowing the LDL to be in hydrated suspension in the blood or extracellular fluids.
The cholesterol is delivered to cells on LDL via a specific LDL receptor, and is liberated from the LDL particles in lysosomes where it can control the cell's cholesterol metabolism. An accumulation of intracellular cholesterol modulates three processes.
First, it reduces the cell's ability to make its own cholesterol by turning off the synthesis of an enzyme, HMG CoA reductase, that catalyzes a step in cholesterol's biosynthetic pathway. Suppression of the enzyme leaves the cell dependent on external cholesterol derived from the receptor-mediated uptake of LDL.
Second, the incoming LDL-derived cholesterol promotes the storage of cholesterol in the cell by activating an enzyme denominated lipoprotein acyltransferase. That enzyme esterifies fatty acids to excess cholesterol molecules, making cholesteryl esters that are deposited in storage droplets.
Third, and most significant, the accumulation of cholesterol within the cell drives a feedback mechanism that makes the cell stop synthesizing new LDL receptors. Cells thereby adjust their complement of external receptors so that enough cholesterol is brought into the cells to meet the cells' varying demands but not enough to overload them. For example, fibroblasts that are actively dividing, so that new membrane material is needed, maintain a maximum complement of LDL receptors of about 40,000 per cell. In cells that are not growing, the incoming cholesterol begins to accumulate, the feedback system reduces receptor manufacture and the complement of receptors is reduced as much as tenfold.
On the other hand, it has been shown that another circulating lipoprotein, high density lipoprotein (HDL) particle is implicated in a state of elevated cholesterol associated with lowered risk of atherosclerosis. Apolipoprotein A-I is a structural protein and antigen of the HDL particle. The amount of HDL provides an inverse correlation with the predicted incidence of atherosclerosis.
High density lipoprotein (HDL) contains two major apolipoproteins, apolipoprotein A-I (apo A-I) and apolipoprotein A-II (apo A-II). Apo A-I is the major protein component of all primate HDL. All HDL particles contain apo A-I, and therefore immunoquantification of HDL has usually involved the quantitation of apo A-I. About 80% of HDL particles also contain apo A-II, but HDL particles containing only apo A-II have not been described.
One function of apo A-I is the activation of the plasma enzyme, lecithin-cholesterol acyltransferase (LCAT). This enzyme is required for the esterification of free cholesterol on HDL for transport to the liver. In the absence of apo A-I, cholesterol in the blood is not esterified and thus cholesterol is not cleared from the blood. The specific role in HDL metabolism served by apo A-II has not been defined.
Many studies have shown that elevated HDL levels correlate with a reduced incidence of CAD. Some authors have speculated that HDL removes cholesterol from peripheral sites, such as the arterial wall, therefore attributing anti-atherogenic properties to HDL. Higher concentrations of HDL cholesterol are correlated with relatively normal lipid metabolism and a lower incidence of and/or a decreased severity of cardiovascular disease, whereas elevated levels of LDL cholesterol are associated with abnormal lipid metabolism and an increased risk of CAD. For the proper management of patients with hyperlipidemia (excess lipids in the blood) and those patients at special risk for CAD, it is desirable to frequently determine levels of LDL and HDL cholesterol. To date, assays of HDL cholesterol have been cumbersome and inaccurate in determining blood levels of HDL.